Method and apparatus for quick acquisition of pilot signals using bank switching method

ABSTRACT

When a mobile communication unit (e.g. a cellular telephone) is powered up, the unit must lock on to a local base station, or “acquire” a base station signal, to enable the user to send and receive calls. To lock on a local base station, the mobile unit must determine the delay at which the base station is sending the pseudo random (PN) code. This process is called the “acquisition.” The current art of acquiring a base station involves collecting a set of samples at a particular code phase, or delay, testing the collected sample, and repeating these steps using another code phase until the correct code phase is found. The present invention discloses a method and apparatus for collecting a set of samples at a particular code phase, and simultaneously testing the collected sample and collecting the next set of samples for another code phase. Using multiple banks, the system resources such as the dwell accumulators and the DSP are used concurrently to reduce the time required to test the phase delays of the short code to lock on to a base station.

This application is a continuation application of U.S. application Ser.No. 08/955,499, filed Oct. 22, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the art of wirelesscommunications. In particular, the present invention relates to the artof searching for the direct sequence spread spectrum pilot signals ofbase stations to establish communication between a mobile unit and thebase station.

2. Description of Related Art

In wireless communications technology, user data (e.g. speech) isencoded in a radio frequency for transmission and reception between abase station and a mobile unit. The radio spectrum allocated byregulatory authorities for a wireless system is “trunked” to allowsimultaneous use of that spectrum block by multiple units.

The most common form of trunked access is the frequency-divisionmultiple access (FDMA) system. In an FDMA system voice is commonlytransmitted through analog modulation but can in principle be digitizedand transmitted with digital modulation. In FDMA, the spectrum isdivided into frequency channels comprised of distinct portions of thespectrum. The limited frequency channels are allocated to users asneeded. However, once a frequency channel is assigned to a user, thatfrequency channel is used exclusively by the user until the user nolonger needs the channel. This limits the number of concurrent users ofeach frequency channel to one, and the total number of users of theentire system, at any instant, to the number of available frequencychannels.

Another common trunking system is the time-division multiple access(TDMA) system. TDMA is commonly used in telephone networks, especiallyin cellular telephone systems, in combination with an FDMA structure. InTDMA, data (speech) is digitized and compressed to eliminate redundancythus decreasing the average amount of bits required to be transmittedand received for the same amount of information. The time line of eachof the frequency channels used by the TDMA system is divided into“frames” and each of the users sharing the common channel is assigned atime slot within the frames. Each user then transmits a burst of dataduring its assigned timeslot and transmits nothing during other times.With the exception of delays required by the bursty data transmission,the TDMA system will appear to each of the users sharing the frequencychannel to have provided an entire channel to each user.

The FDMA and TDMA combination technique is used by the GSM (globalsystem for mobile communications) digital cellular system. In GSM, eachchannel is divided up in time into frames during which eight differentusers share the channel. A GSM time slot is only 577 μs (micro-seconds),and each users gets to use the channel for 577 μs every 4.615 ms(milli-seconds). 577 μs*8 =4.615 ms.

Yet another method for sharing a common channel between multiple usersis the code-division multiple access (CDMA) technique using directsequence spread spectrum modulation. CDMA is relatively new to thecellular technology and is one of the accepted techniques to be includedinto the next generation of digital cellular systems in the UnitedStates of America (U.S.A.).

As with TDMA, the CDMA systems are typically used in conjunction with aFDMA structure, although this is not required. However, unlike the TDMAsystem, the CDMA system does not separate the multiple users of a commonfrequency channel using time slices. Rather, in CDMA, multiple users areseparated from each other by superimposing a user-specific high-speedcode on the modulation of the data of each user. Because the separatingcode has the effect of spreading the shared channel of each user'stransmission, the CDMA system is often called a “spread spectrum”system. “Direct sequence” spreading is accomplished by multiplying anarrowband information carrying signal by a much wider band spreadingsignal. The error coded and digitally modulated data (speech) for eachof the shared users of the CDMA channel may typically be 9.6, 14.4, or19.2 KHz wide. This is spread using a much wider spreading signal whichmay be 1.2288 MHZ wide. Using the wider spreading signal, a CDMAfrequency channel can accommodate many users on code sub-channels. Thespreading signal is usually a sequence of pseudo random bits (PN code)and is often called a “spreading code,” or “chipping code” because it“spreads” or “chips” the much slower data bits. The PN code is differentfor differing users, allowing a user to distinguish its code sub-channelfrom other users' sub-channels on the same frequency channel. The PNsequence may be expressed as c(t), where the chipping function, c( ), isa function of time t. The PN sequence is generated using a linearfeedback shift register (LFSR) which outputs a random-like sequence ofdigital ones and zeros. These digital ones and zeros are modulated to −1and +1 respectively and filtered to give the chipping function c(t).Thus the chipping function has the property that c(t)²=+1. The PNsequence generated by a N-register LFSR is 2^(N)−1 chips long, though acommon system artificially inserts a zero to extend the full sequencelength to 2¹⁵=32768 chips. That system has a chip rate of 1.2288 MHz, sothat the sequence repeats every 26.666 ms.

In a typical system, each base station maintains a pilot channel withits own identifying spreading code for the mobile units to refer to. Apilot signal is a spread signal with no underlying informationmodulation, such that the exact waveform is known by both transmitterand receiver, with the exception of the waveform timing. The mobileunits use the pilot channel to synchronize themselves with the basestation so they can effectively communicate with the base station. Whena mobile unit is powered on, the mobile unit initially searches for apilot channel in an attempt to establish a lock with a base station.This process is called “acquisition.” In order to “acquire,” or lock on,to a base station, the mobile must align its locally generated versionof the PN sequence with the PN sequence of the base station bydetermining the timing of the transmitted pilot's spreading sequence.The present invention provides for an improved acquisition technique.

At power up, a mobile unit must search for a pilot to synchronize itsspreading sequence with that of a base station. The acquisition processis generally described using the system as illustrated by FIG. 1. FIG. 1is a simplified diagram illustrating the major functions of a systemwhich can implement the acquisition process.

In the simplified model of FIG. 1, the radio signal is received by anantenna 12. The signal at line 14 is a radio frequency signal which isabout 800 to 900 MHz for cellular communications. The signal at line 14,S₁₄, can be expressed as

 S₁₄ =d(t)c(t-D _(base)) cos (t)

where

d(t) is the data (speech in digitized form);

c(t-D_(base)) is the PN short code at delay D_(base) which is the basestation delay; and

cos (t) is the radio frequency carrier wave.

Of course, c(t-D_(base)) is the spreading code sequence used in the CDMAsystem, and would not be present in a non-CDMA system. A pilot signalcontains no data, so in the case of a pilot signal d(t)=1 and isconstant. The pilot signal spreading code is a different PN code fromthe data spreading code, allowing the two signals to be distinguished.Once the pilot code timing is known, that same timing can be applied tothe data spreading code to allow the receiver to demodulate the digitaldata.

The process of acquisition, then, is the process of determining thevalue of D_(base). Once the value of D_(base) is determined, the mobilecan use the same c(t-D_(base)) sequence to lock on to the base signaland remove the spreading code to retrieve the data, d(t).

The quadrature demodulator circuit 16 removes the carrier wave portion,cos(t), from the incoming RF signal and provides a complex valuedbaseband signal to the sampling circuit 20 which converts the analog RFinto digital samples at the spread spectrum frequency of 1.2288 MHZ. Atline 22, the signal can be expressed as

S₂₂ =d(t)c(t−D _(base))

The base station delay, D_(base), is not known by the mobile unit atpower up. If D_(base) is known, then the PN code delay at the mobileunit, D_(mobile), can be set to match D_(base), and S₂₂ can bemultiplied by c(t-D_(mobile)) to eliminate the spreading sequence toretrieve the data. Alternatively expressed, if D_(mobile)=D_(base), then$\begin{matrix}{{{d(t)}{c\left( {t - D_{base}} \right)}{c\left( {t - D_{mobile}} \right)}} = \quad {{d(t)}{c\left( {t - D_{base}} \right)}{c\left( {t - D_{base}} \right)}}} \\{= \quad \begin{matrix}{d(t)} & {;\quad {{\text{because}\quad {c(t)}^{2}} = 1}}\end{matrix}}\end{matrix}$

Fixed Dwell Search System (FDSS)

Assuming that N=15 such that the full sequence length is 2¹⁵, at powerup, D_(base) is not known, and the mobile must test each of the 2¹⁵possibilities to find D_(base). In the Fixed Dwell Serial Search (FDSS)systems, D_(base) is found by brute-force, trial and error method whichcan be outlined as follows (continuing to refer to FIG. 1):

1. The incoming signal is multiplied by a multiplier 24 by a PN codewith an initial delay, D_(test) 44.

2. The result of the multiplication is summed, or accumulated 28, for Nnumber of chips, N being a predetermined number of chips. The process ofmultiplying and accumulating for a period of time is referred to as adwell or a dwell period. The pilot signal could be thought of as beingconstructed based on a sequence of zero's (0) and one's (1). It iscommon in the industry to refer to each digit of a digital spreadingsequence as a “chip.” For example, a digital spreading signal of a fixedduration containing 100 digital values can be called a set of 100 chips.

3. The energy of the accumulated value of the products are calculated 34by taking a magnitude squared of the accumulant.

4. The calculated energy is compared to some pre-set threshold, γ, 38.

5. And, a determination is made. If the calculated energy equals orexceeds the threshold value, γ, then the given delay being tested,D_(test), 44 is determined to be a potential signal and is verified. Ifthe verification is successful, then D_(test) is determined to equalD_(base) and the acquisition terminates. If the calculated energy isless than the threshold value, then D_(test) 44 is not equal toD_(base), and the next delay value is tested beginning at step 1. Infact, the delay value is tested at every ½ chip. Therefore, the numberof delays tested is 2*2¹⁵, or 2¹⁶.

The multiplication (step 1 above) and the summation (step 2) aretypically done using a specially designed hardware, and is performed atthe same speed as the incoming chip rate. The energy calculation (step3) and the comparison with a threshold (step 4) could be performed insoftware by a digital signal processor (DSP) 42 as indicated by the dashline in FIG. 1.

If the incoming signal at line 22, S₂₂, is multiplied by the correctlydelayed PN code, then the sum, or integration, of the energy levels of aset of chips will add up to a signal strength approaching some amplitudevalue, A. If the incoming signal at line 22, S₂₂, is multiplied 24 by anincorrectly delayed PN code 44, then the signal at line 26, S₂₆, willappear as noise and the sum 28, or integration, of the energy levels ofthe set of chips will approach zero. The DSP 42, controls the value of D44, to increment the phase based on the dwell/threshold decisions. This“de-spreading” method is discussed in many text and reference books. Forexample, see Redl, et. al., AN INTRODUCTION TO GSM, pp. 61-63. Inreality, however, the results of the integration do not fall exactly atA or exactly at 0, but are corrupted by noise and fall near A or near 0,and appear as some probability function near A or 0.

The Multiple Dwell Search (MDS) method is based on the facts that themobile unit needs to find the base station delay, D_(base), and that theaverage acquisition time, T_(aa), can be minimized by quickly rejectingall other delays. To that end, first, a smaller sample (from arelatively shorter dwell period) is tested against D_(test) to filterout obvious noise. If the sample fails the first test, then D_(test), isrejected. On the other hand, if the sample passes the test, thenD_(test) is further tested by collecting another, larger set of samples.This process is more efficient than the simple FDSS because most of thedelays are rejected using a smaller sized set of chips. The MDStechnique achieves a quicker acquisition by decreasing the average dwelltime portion of T_(aa). However, the MDS technique does not address thecomputational time portion of T_(aa). In fact, because T_(aa) is a sumof the average dwell time and average computation time, or,

T_(aa)=KT_(average) _(—) _(dwell) _(—) _(time)+KT_(average) _(—)_(computational) _(—) _(time)

where

T_(average) _(—) _(dwell) _(—) _(time) is average time period the mobileunit dwells at each delay;

T_(average) _(—) _(computational) _(—) _(time) is the average timeperiod the mobile unit takes to calculate the energy from a dwell andcompare it to a predetermined threshold; and

K is the sample range, or the number of possible delays that must besearched, and is set to 2¹⁶, not 2¹⁵ because the delay is tested at ½the chip increments.

Any decreases in the T_(average) _(—) _(dwell) _(—) _(time) portion ofthe equation increases the prominence in, or the ratio of theT_(average) _(—) _(computational) _(—) _(time) in the overall T_(aa)determination.

In summary, because the dwelling and the computational steps of theacquisition processes are serially performed, the average acquisitiontime, T_(aa), is unnecessarily increased by 25% to 33% in the FDSSsystem. The negative impact of the computational step to the overallT_(aa) is even larger for other more efficient acquisition techniquessuch as the MDS technique.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to eliminate the timerequired to calculate the energy of an incoming pilot signal and compareit with a threshold by implementing these steps to be performedsimultaneously with the dwelling step.

The present invention provides a method of synchronizing a cellular unitto a base station pilot signal, each base station's pilot signal beingspread by a pseudo-random noise (PN) code having and a signal strength.A predetermined number of chips of the incoming signal is multiplied bya PN code having a first delay, and the energy accumulated. This iscalled “dwelling” at first delay. Then, the accumulant is processed,which will include a threshold comparison while the mobile unit dwellsfor a second set of chips at a second delay of the PN code. If desired,the first delay can be returned for further measurements.

An alternative summary of the present invention is a method of selectinga base station delay by using two banks. The first bank uses thehardware and software resources to analyze (dwell and compare) a firstdelay, and the second bank operates identically except that the secondbank uses the resource not being used by the first bank at any oneinstant in time. That is, the system resources can be assigned and usedsimultaneously if they are assigned to different banks. For example, thefirst bank may be dwelling at a first phase while the second bank may becomparing its accumulant.

The present invention also provides for a mobile communicationsapparatus comprising a dwell accumulator resource, a digital signalprocessor (DSP) resource, and two banks. The resources and the banks areall interconnected to work together. The banks alternate to efficientlyuse the resource not being used by the other bank. Typically, a bank isa memory location which stores the data necessary for testing a givencode phase and controls resources of the system in order to perform thattesting.

Also provided for in the present invention is a machine-readable storagemedium containing instructions for a processor to perform the techniquesof the present invention.

These and other aspects, features, and advantages of the presentinvention will be apparent to those persons having ordinary skilled inthe art to which the present invention relates from the foregoingdescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrating the major functions ofa direct sequence spread spectrum mobile communications unit;

FIG. 2 is a flowchart illustrating the present invention; and

FIG. 3 is a block diagram illustrating an apparatus according to apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 3, a mobile cellular unit according to the presentinvention is shown. Similar to the prior art unit of FIG. 1, the unitshown in FIG. 3 receives the pilot signal using an antenna 12, removesthe radio carrier wave 16, and digitizes the signal 20. Then, thedigitized signal is multiplied by a multiplier 24, the product isaccumulated by an accumulator 28, and the accumulated energy is comparedwith a threshold by a DSP 42. The DSP 42 may square 34 the accumulantprior to comparing 38 the accumulant with the threshold.

The digitizer 20, the multiplier 24, and the accumulator 28,collectively, is used to “dwell” for N chips to accumulate its amplitudewhere N is a predetermined number of chips. The “dwell” operationcomprises the steps of digitizing N chips 20, multiplying the chips tothe mobile unit's pilot using a test delay 24, and accumulating theresults of the multiplications 28. The collection of these circuits orthese functions is referred to as a “dwell accumulator resource” 72. Theportion of the DSP 42 which squares the accumulant 34 to generage theenergy and compares the energy to a threshold 38 is collectivelyreferred to as a “DSP resource” 74.

In the prior art, these resources were used in a serial manner. That is,the dwell accumulator resource 72 is used while the DSP resource 74waited for the dwell accumulator resource 72 to finish its operations.Then, the DSP resource 74 is used while the dwell accumulator resource72 waited for the DSP resource 74 to finish its operations.

In the present invention, two banks 76 and 78 are provided. Forsimplicity, the banks are referred to as bank A 76 and bank B 78. Eachof the banks 76 and 78 are connected to each of the resources 72 and 74and interfaces with them. In an actual implementation, the banks aretypically memory locations. In the present implementation, the banks areused to control the operations of the resources to allow the resourcesto operate simultaneously, or in parallel. Typically, a bank is a memorylocation which stores the data necessary for testing a given code phaseand controls resources of the system in order to perform that testing.

The operations of the banks 76 and 78 with the resources 72 and 74 canbe illustrated using the flowchart of FIG. 2.

Referring now to FIG. 2 but continuing to refer to FIG. 3, the initialstep of the present invention technique is setting of the initial testdelays for banks A and B. This operation is referred to by block 60 ofthe flowchart of FIG. 2. Typically, this operation involves setting thetest delay for bank A 76, D_(a), to 0 and the test delay for bank B 78,D_(b), to 1. Then, one of the banks is given control of the dwellaccumulator resource 72. In this example, bank A 76 is given the controlof the resource 72 first, and the resource 72 dwells for N chips at thedelay of D_(a) as indicated by block 61.

After the dwell 61, bank A 76 is given control of the DSP resource 74 inorder to test the accumulant. This operation is indicated by block 62.At the same time that bank A 76 is using the DSP resource 74, bank B 78is given control of the dwell accumulator resource 72 to allowaccumulation of the chips using the D_(b) test delay. This operation isindicated by the block 63. Likewise, when bank B 78 is utilizing the DSPresource 74, bank A 76 is utilizing the dwell accumulator resource 72.In other words, operations represented by blocks 65 and 66 aresimultaneous and the operations represented by blocks 62 and 63 aresimultaneous.

Using this technique, the resources are efficiently used because neitherof the resources is waiting for the other resource to complete itsoperation. In FIG. 2, the operations represented by the blocks withindashed-block 69 is performed using bank A and the operations representedby the blocks within dashed-block 70 is performed using bank B.

As indicated by blocks 62 and 67, the resultant accumulant of a dwellusing a delay value is tested by comparing the accumulant to one or morepredetermined threshold. In a preferred embodiment, the accumulant has areal component and an imaginary component. Thus, the accumulant ismagnitude squared to remove the imaginary component and obtain theenergy of the accumulant before being tested.

If the energy level is greater than a predetermined threshold (“acceptthreshold”), the process is halted and the delay, or the phase, of thecode used to despread the chips for said accumulant is considered to bethe base station delay. This operation is represented by block 68. Ifthe energy level is less than another threshold (“reject threshold”),then the phase is rejected, and another phase is tested. If the energylevel is indeterminate as to whether the tested phase is the phase ofthe base station, then the same phase can be tested again.

The accept threshold level is predetermined such that if the accumulantenergy level exceeds the first threshold, then the tested code phase isthe base station phase. The reject threshold level is predetermined suchthat if the accumulant energy level is less than the second, then thetested code phase can be rejected.

When the energy of the accumulant of bank A 76 or of the accumulant ofbank B 78 exceeds the accept threshold, the process is halted and thedelay, or the phase, of the code used to despread the chips for saidaccumulant is considered to be the base station delay. This operation isrepresented by block 68.

When the energy of the accumulant of bank A 76 or of the accumulant ofbank B 78 is less than the reject threshold, then the tested code phaseis rejected and another phase is tested. This is accomplished by settingthe test delay value to another, new value and dwelling for another Nchips. These operations are represented by the blocks 64 for D_(a) and67 for D_(b). Typically, to change the delay value, the value isincremented or added to the delay value of the other bank. That is, whenD_(a)=0 and D_(b)=1, then the next set of delay values tested areD_(a)=2 and D_(b)=3,and so on. However, the two test delay values aretested independently of each other.

When the energy of the accumulant of bank A 76 or of the accumulant ofbank B 78 is indeterminate as to whether the tested phase is the phaseof the base station, then the same phase can be tested again. This isaccomplished by keeping the same code phase delay for another dwell-testcycle.

The delay value is not required to change between dwells. For instance,if the accumulant at D_(a)=5 leads to an inconclusive result as to thebase station delay, then D_(a) of 5 can be maintained to accumulate morechips for another dwell period, using the same delay value.

For example, assume that D_(a)=0 and D_(b)=1 for a particular dwellperiod. If the energy of the incoming signal at delay value of 0 doesnot meet the threshold, then D_(a) is set to 2 for the next dwell cycle.On the other hand, if the energy of the incoming signal at delay valueof 1 may or may not meet the threshold, then the value of D_(b) is notchanged to test, again, the incoming signal at delay 1. That is, the newvalue of D_(b) is same as the old value.

Because an accumulant is being tested simultaneously with the nextdwelling step, the time is used efficiently. Alternatively expressed,because T_(average) _(—) _(dwell) _(—) _(time) and T_(average) _(—)_(computational) _(—) _(time) are performed simultaneously, the newT_(aa) is the larger, not the sum, of the two figures, or

T _(aa)=max(T_(average) _(—) _(dwell) _(—) _(time), T_(average) _(—)_(computational) _(—) _(time))

The banks are controlled by the DSP, and the instructions for the DSP tocontrol multiple banks may be stored in any machine readable storagemedium such as a semiconductor memory device, magnetic device, opticaldevice, magneto-optical device, floppy diskette, hard drive, CD-ROM,magnetic tape, computer memory, and memory card.

The present invention and adaptations of the present invention may beimplemented in combination with other signal acquisition techniques. Inparticular, the present invention is well suited to be implemented withthe modified multiple dwell search technique as disclosed by a patentapplication Ser. No. 08/956,056 entitled “METHOD AND APPARATUS FORMULTI-DWELL SEARCH OF PILOT SIGNAL IN A CDMA COMMUNICATION SYSTEM.” Theentire patent application Ser. No. 08/956,056 entitled “METHOD ANDAPPARATUS FOR MULTI-DWELL SEARCH OF PILOT SIGNAL IN A CDMA COMMUNICATIONSYSTEM,” having three inventors—Roland Rick, Brian Banister, and MarkDavis—and being filed currently with the present patent application, ishereby incorporated in full into the present application.

Although the present invention has been described in detail with regardto the exemplary embodiments and drawings thereof, it should be apparentto those skilled in the art that various adaptations and modificationsof the present invention may be accomplished without departing from thespirit and the scope of the invention. Accordingly, the invention is notlimited to the precise embodiment shown in the drawings and described indetail hereinabove. Therefore, it is intended that all such variationsnot departing from the spirit of the invention be considered as withinthe scope thereof as limited solely by the claims appended hereto.

In the following claims, those elements which do not include the words“means for” are intended not to be interpreted under 35 U.S.C.§112¶ 6.

What is claimed is:
 1. A method of synchronizing a cellular unit to abase station signal, each base station's direct sequence spread spectrumpilot signal being spread by a pseudo-random noise (PN) code and havinga signal strength, said method comprising: dwelling for a first set ofchips at a first delay of the PN code to produce an accumulant which isan accumulation of values; and comparing said accumulant to a thresholdwhile dwelling for a second set of chips at a second delay of the PNcode, wherein said step of dwelling for the first set of chips and saidstep of dwelling for the second set of chips are performed in a sameprocessing path.
 2. A method according to claim 1 further comprising thestep of magnitude squaring said accumulant prior to comparing it to saidthreshold.
 3. A method according to claim 1 wherein said step ofdwelling comprises the steps: multiplying a received signal with the PNcode having said, first delay to obtain a sequence of products; andaccumulating said products.
 4. A method according to claim 1 whereinsaid threshold is predetermined to distinguish signal from noise.
 5. Amethod according to claim 1, wherein a same apparatus used to performsaid step of dwelling for the first set of chips is also used to dwellfor the second set of chips while said accumulant is compared to thethreshold.
 6. A method of synchronizing a cellular unit to a basestation signal, each base station's direct sequence spread spectrumpilot signal being spread by a pseudo-random noise (PN) code and havinga signal strength, said method comprising the steps of: determiningvalues for a first delay phase and a second delay phase; dwelling for apredetermined number of chips at said first delay phase to produce afirst accumulant; comparing said first accumulant to a threshold;dwelling, simultaneous with said comparison of said first accumulant tosaid threshold, for said predetermined number of chips at said seconddelay phase to produce a second accumulant; determining a new value forsaid first delay phase; comparing said second accumulant to saidthreshold; dwelling, simultaneous with said comparison of said secondaccumulant to said threshold, for said predetermined number of chips atthe new value of said first delay phase to produce a new value for saidfirst accumulant; and determining a new value for said second delayphase, wherein said steps of dwelling to produce the first accumulant,the second accumulant and the new value for the first accumulant areperformed in a same processing path.
 7. A method according to claim 6further comprising a step of repeating said steps of claim 5 until saidfirst accumulant is greater than said threshold.
 8. A method accordingto claim 6 further comprising a step of repeating said steps of claim 5until said second accumulant is greater than said threshold.
 9. A methodaccording to claim 6 wherein said step of determining a new value forsaid first delay comprises a step of incrementing said first delay. 10.A method according to claim 6 wherein said step of determining a newvalue for said first delay comprises a step of assigning to said firstdelay an incremented value of said second delay.
 11. A method accordingto claim 6 wherein said first accumulant is stored in a first bank. 12.A method according to claim 6 further comprising a step of magnitudesquaring said first accumulant prior to comparing it to said threshold.13. A method according to claim 6 wherein said step of dwellingcomprises the steps: multiplying a received signal with the PN codehaving a delay to obtain a sequence of products; and accumulating saidproducts.
 14. A method according to claim 6 wherein said threshold ispredetermined to distinguish signal from noise.
 15. A method accordingto claim 6, wherein said steps of dwelling to produce the firstaccumulant, the second accumulant and the new value for the firstaccumulant are performed by a same apparatus.
 16. A machine-readablestorage medium containing instructions for a processor, saidinstructions for synchronizing a mobile communications device to a basestation signal having a base delay, said instructions comprising stepsto: a. dwell for a first set of chips at a first delay of the PN code toproduce an accumulant which is an accumulation of chip magnitudes; andb. compare said accumulant to a threshold while dwelling for a secondset of chips at a second delay of the PN code, wherein said step todwell for the first set of chips and said step to dwell for the secondset of chips are performed in a same processing path.
 17. A storagemedium according to claim 16 wherein said storage medium is selectedfrom a group consisting of semiconductor memory device, magnetic device,optical device, magneto-optical device, floppy diskette, hard drive,CD-ROM, magnetic tape, computer memory, and memory card.
 18. Amachine-readable storage medium according to claim 16, wherein said stepto dwell for the first set of chips and said step to dwell for thesecond set of chips are performed by a same apparatus.
 19. An apparatusfor synchronizing a cellular unit to a base station signal, each basestation's direct sequence spread spectrum pilot signal being spread by apseudo-random noise (PN) code and having a signal strength, saidapparatus comprising: means for dwelling for a first set of chips at afirst delay of the PN code to produce an accumulant which is anaccumulation of values; and means for comparing said accumulant to athreshold while dwelling for a second set of chips at a second delay ofthe PN code, wherein dwelling for the first set of chips and dwellingfor the second set of chips are performed in a same processing path. 20.A method of synchronizing a cellular unit to a base station signal, eachbase station's direct sequence spread spectrum pilot signal being spreadby a pseudo-random noise (PN) code and having a signal strength, saidmethod comprising the steps of: determining values for a first delayphase and a second delay phase; dwelling for a predetermined number ofchips at said first delay phase to produce a first accumulant; comparingsaid first accumulant to a threshold; dwelling, simultaneous with saidcomparison of said first accumulant to said threshold, for saidpredetermined number of chips at said second delay phase to produce asecond accumulant; determining a new value for said first delay phase;comparing said second accumulant to said threshold; dwelling,simultaneous with said comparison of said second accumulant to saidthreshold, for said predetermined number of chips at the new value ofsaid first delay phase to produce a new value for said first accumulant;and determining a new value for said second delay phase, wherein saidstep of determining a new value for said first delay comprises a step ofassigning to said first delay an incremented value of said second delay.21. An apparatus for synchronizing a cellular unit to a base stationsignal, each base station's direct sequence spread spectrum pilot signalbeing spread by a pseudo-random noise (PN) code and having a signalstrength, said method comprising the steps of: (a) means for determiningvalues for a first delay phase and a second delay phase; (b) means fordwelling for a predetermined number of chips at said first delay phaseto produce a first accumulant; (c) means for comparing said firstaccumulant to a threshold; (d) means for dwelling, simultaneous withsaid comparison of said first accumulant to said threshold, for saidpredetermined number of chips at said second delay phase to produce asecond accumulant; (e) means for determining a new value for said firstdelay phase; (f) means for comparing said second accumulant to saidthreshold; (g) means for dwelling, simultaneous with said comparison ofsaid second accumulant to said threshold, for said predetermined numberof chips at the new value of said first delay phase to produce a newvalue for said first accumulant; and (h) means for determining a newvalue for said second delay phase, wherein determining the new value forsaid first delay comprises a step of assigning to said first delay anincremented value of said second delay.
 22. An apparatus forsynchronizing a cellular unit to a base station signal, each basestation's direct sequence spread spectrum pilot signal being spread by apseudo-random noise (PN) code and having a signal strength, said methodcomprising the steps of: (a) means for determining values for a firstdelay phase and a second delay phase; (b) means for dwelling for apredetermined number of chips at said first delay phase to produce afirst accumulant; (c) means for comparing said first accumulant to athreshold; (d) means for dwelling, simultaneous with said comparison ofsaid first accumulant to said threshold, for said predetermined numberof chips at said second delay phase to produce a second accumulant; (e)means for determining a new value for said first delay phase; (f) meansfor comparing said second accumulant to said threshold; (g) means fordwelling, simultaneous with said comparison of said second accumulant tosaid threshold, for said predetermined number of chips at the new valueof said first delay phase to produce a new value for said firstaccumulant; and (h) means for determining a new value for said seconddelay phase, wherein determining the new value for said first delaycomprises a step of assigning to said first delay an incremented valueof said second delay, wherein said dwelling to produce the firstaccumulant, the second accumulant and the new value for the firstaccumulant are performed in a same processing path.